The present invention was conceived to measure forces occurring at the interface between a vehicle tire and the road, specifically, the vertical force and the force and aft forces parallel to the longitudinal axis of the vehicle. This force measuring environment creates very severe practical problems. The transducer is normally located along the axis of the tire, while the force inputs are at the road surface. Thus, the forces to be measured are in fact applied at a distance from the transducer axis equal to the tire radius, creating very substantial bending moments at the location of the transducer. Additional bending moments, as well as extraneous forces, are created by lateral skid forces parallel to the axis of tire rotation.
Still another practical problem is the size of the transducer structure. The presence of the wheel prevents the transducer from being located directly above the road contacting portion of the tire. Instead, it must be located inboard therefrom, thus creating another moment arm. It is therefore desirable to make the transducer structure as small as possible, so that its inboard displacement can be minimized.
The combination of large extraneous forces and moments and compact construction results in high stress levels and extraneous error signals in the transducer structure. While electrical compensation techniques have been used in the past for correcting the extraneous signals produced by the strain gages, it has been difficult to obtain reasonable signal levels for the forces to be measured while suppressing signals from extraneous forces.
It is therefore the principal object of the present invention to provide a transducer capable of measuring forces along two mutually perpendicular axes, while being insensitive to extraneous forces and moments, strong enough to provide low and safe stress levels, and small in overall size.